Why is the skin darker around the nipple?
The pigmented area around the nipple is known as the areola. It usually
has a darker color than the surrounding skin. That means that in people
with dark skin, the areola is darker still. The areola also goes through a
number of color changes due to blood flow to this region. The areola can
very lightly tinted in a woman who has never given birth. During pregnancy
(starting about the second month) the areola darkens with the increased
blood flow to the breasts that are preparing for lactation. After the
woman gives birth, and completes breast feeding, the areola may lighten
but its color usually does not completely return to the previous lighter
shade.
Now, as to why the color difference? I don’t know that there is a
definitive reason. The areola is not just skin, it is a different tissue
type. As such, some of its color difference is probably due to basic
tissue differences. This is probably the best reason why the areola is
visible in males.
A common explanation for the areola is that it provides a “target” for the
nursing infant to find the nipple. While I cannot say that this
explanation is wrong, I find that it is unsatisfactory. A new born infant
has very little visual acuity, yet can find the nipple without difficulty.
Similarly, babies that are bottle fed find the nipple of the bottle
without the aid of any visual target. Finally, no other mammal has a
visual cue to identify the nipple, so why should humans have it? So, I
don’t particularly like the “target” theory.
A more satisfactory explanation, in my mind, is that the areola provides a
visual clue as to sexual reproductive success. Visual signals for sexual
receptivity and reproductive success are common among animals. Humans are
unusual in that the female seems to have no obvious signal for
reproductive success or sexual receptivity. When this type of signal
appears in every other mammal, but not in humans we have to ask why (and
this is a long debated topic in human evolutionary biology). One answer to
this question may be that human females do provide signals, but human
culture has obfuscated them. The fact that the areola becomes darker
following a successful pregnancy is a signal of reproductive success. As
such, it could provide a selective advantage in the evolutionary sense.
The “sexual signal” theory has not be tested in any meaningful way, and so
I cannot state that this is the actual reason why the areola is darker
than the surrounding skin. However, this theory does make sense to me as a
human biologist, and so I will claim that it is the best explanation that
is currently available.